We have studied the regulation of DNA gyrase, the bacterial enzyme responsible for DNA supercoiling. Synthesis of both the A and B subunits of DNA gyrase in E. coli is itself controlled by DNA supercoiling, with much more enzyme being made from a relaxed DNA template than from a supercoiled one. We have also shown that topoisomerase I, whose DNA-relaxing activity balances the supercoiling produced by gyrase, is controlled in the opposite way; its synthesis is greatly reduced when DNA is relaxed. Each enzyme is thus preferentially synthesized when more of it is needed, enabling the cells to maintain control over the level of DNA supercoiling. In order to study this mechanism of regulation in more detail, we have identified the promoter regions of the gyrA and gyrB genes, and have fused each promoter to the galactokinase gene in a plasmid vector. In these hybrid plasmids, which contain about 100 base pairs of the region surrounding the promoter, galactokinase synthesis is strongly stimulated by DNA relaxation. Thus the small region around the gyrA or gyrB promoter contains all the necessary information for this type of control. A deletion analysis has identified the necessary sequences even more closely, and is being pursued in order to locate the regulatory sites exactly.